This invention relates to an electric power generation system and, more particularly, to a method and system for disconnecting a distributed power generation unit from a power distribution system.
An electric power system of a country, such as the United States, typically consists of large, regional power systems interconnected to form an electric power grid or circuit. Primary electric power in any region is provided by a government-regulated electric utility company, which may be investor owned, membership owned, or government owned. A regional grid is dynamic and may comprise a plurality of distributed generation units, or producers of electrical power that are directly connected to a power distribution system or grid that is privately owned and maintained by entities other than the electric utility company. For example, some manufacturers, or even individuals, may have their own power generation equipment and may at times have excess capacity which, by law, can be sold to the power utility company. For this reason, electric power grids are growing increasingly complex and interconnected, with a greater number of power buyers and sellers making a burgeoning number of transactions. Distributed generation units may be conventional generators driven by reciprocating engines, turbine engines, micro turbines, water turbines or wind turbines. Distributed generation units may also be energized by fuel cells, photovoltaic panels, or other energy conversion technologies.
The larger distributed generation units are typically interconnected in parallel with an existing electric utility grid through radial distribution circuits at distribution voltage levels. Other smaller distributed generation units interconnect in parallel with the electric utility grid on a secondary, or low voltage side of a transformer.
Safety of the general public is greatly compromised when an overhead distribution or transmission line conductor falls to the ground. When such a condition exists, power must be quickly interrupted from all sources supplying power to the downed conductor. The primary electric utility company has control over power flowing from its transmission system and can interrupt this power, typically by opening a circuit breaker. However, the primary electric utility company may not have control over other distributed generation units which may be coupled to the downed conductor or to the power distribution lines downstream of the downed or broken conductor. Consequently, the grid in the area of the downed conductor may remain energized even though the primary power to the area has been interrupted. Thus, the electric utility, which owns and operates the downed distribution line, may not have the necessary control over de-energizing, or shutting down the flow of electricity from or through, all sources of power connected to the distribution line.
During certain maintenance and construction activities, electric line crews need to know, with certainty, that an electric power distribution line is de-energized and cannot be re-energized without their prior knowledge. When a source of electric power is from a single point, such as with a traditional electric system, line crews can easily isolate the circuit and safely perform the necessary work. However, when distributed generation units are connected to supply electrical power, line crews need to be alert to the possibility of energization of a circuit otherwise thought to be de-energized, due to the presence of electrical power from a distributed generator unit.
Additionally, while performing line maintenance, crews sometime need to be able to positively identify each phase of the 3-phase AC system, at any point along a distribution line. Since there is no physical difference between the different phase conductors, no simple visual identification is possible. Line maintenance crews currently use elaborate methods of tracing the orientation of the conductors from the source substation, and when physical tracing is not possible, such as when sections of the lines are underground, the crew must depend on rules of construction, such as identification based on orientation from top to bottom or left to right of the distribution line.
When a temporary fault occurs on a distribution circuit, power must be interrupted from all sources at effectively the same time in order for the fault to be cleared and service restored. Standard electric utility practice accomplishes this objective automatically through control equipment which senses the fault condition, de-energizes the circuit, typically by opening a substation circuit breaker which interrupts power flow from the only source, and re-energizes the circuit when the fault has cleared, usually by automatically closing the circuit breaker. If the circuit includes distributed generation units, those remote sources of energy may continue to feed the fault condition unless there is control circuitry at each various distributed generation unit to detect the fault condition and/or the isolated condition of the circuit, and execute a shutdown or disconnect. The electric utility, which is responsible for the circuit's quick and reliable return to service, has been dependent on the designers and/or operators of the distributed generator units to be able to detect the abnormal condition and to react accordingly.
Once a power outage occurs in an electric power grid, restoration may be a difficult and lengthy process, possibly requiring many hours. Accordingly, control of many grid operating parameters is carefully monitored, and maintained rigorously by agreement of the system operators and various regulatory agencies. Examples of these parameters include system voltage levels, system frequency, and power flows on transmission lines, all which are affected by the output and location of distribution generator units. While increases in power generated by distributed generation units can be communicated through normal means, a need sometimes arises for a grid operator to reduce or shutdown a distributed generation unit abruptly, yet in an orderly manner, to prevent overloading of lines, overfrequency, overvoltage, or other conditions which can cause or exacerbate major system disruptions. If a significant portion of the generation of the interconnected system comes from distributed generation units, conventional control and communication channels may not be adequate for the required control.
When the control system of the distributed generation unit must rely on detection of certain anomalous conditions of the utility distribution line before recognizing the necessity of immediate isolation from the circuit, it is often too late, and the conditions too severe, to maintain the normal output of the distributed generation unit or generator. The distributed generation unit must shut down, and wait a period of time, which may range from several minutes to hours, before restarting the generator for the purposes of supplying emergency power to the facility. If the owner of the distributed generation unit depends on this generator for emergency power supply when the normal utility supply is disrupted, this sequence is operationally undesirable, and may cause economic hardship, such as loss of product or services.